RT9206GS datasheet(19/21 Pages) RICHTEK

19 / 21 page

RT9206

DS9206-11

March 2007

www.richtek.com

Step2. Determine the zero crossover frequency and

compensated type.

Select desired zero-crossover frequency :

Select fC = 20kHz

Step3. Determine desired location of poles and zeros for

type2 compensator.

Select:

Assume

Step4. Calculate the real parameters-resistor and

capacitors for type2 compensator.

From equation (21), the RC1 is calculated as following :

The design procedure as following :

(1). Selecting the zero crossover frequency fC is 1/10 ~

1/20 switching frequency. Then according equation (24)

set the resistor RC1 to determine the zero crossover

frequency.

(

Ω)

(24)

(2). Place the zero of compensator is 70% fp that is

resonance frequency of power stage. The compensator

capacitor Cc1 can be selected to set the zero. The

equation is shown in following :

××

≈×

××

OUT

REF

0.7R

≈

××

fs-

Design example

Design example of type 2 compensator: the schematic is

shown in Figure 4, where the parameters as following:

VIN=12V,

VOUT=5V,

IOUT=5A,

switching

frequency=200kHz, L=15

µH, CO=940µF, rC=22mΩ, the

parameters of RT9206 as following: gm=1.6ms, ramp

amplitude=1.9V,and reference voltage Vref=0.8V.

Step1. Determine the power stage poles and zeros. The

pole caused by the output inductor and output capacitor is

calculated as :

µµ

πµ

××

× ×

××

f =

= 1.34kHz

L C

940

f =

= 7.7kHz

22m

Ω 940F

(3). Set a second pole to suppress the switching noise.

Assume the pole is one half of switching frequency

fs, which results in capacitor Cc2 as shows in following:

(F) (26)

≤

f /10 ~ f /20

××

CZP

= 0.7

f = 0.7 1.34kHz = 938Hz

= 100kHz

××

×Ω

Ω ××

O UT

R EF

V rV

gmV

20kH z

15 H

1.9

= 8.4k

22m

12V

1.6m s

0.8V

µµ

××

L C

940

= 20.7nF

0.7 R

0.7 8.2k

Select CC1 = 22nF

Second capacitor CC2 can be calculated using equation

(26)

Select CC2 = 220pF

Select RC1 = 8.2k

Ω

Calculate CC1 from equation (25)

(F)

(25)

194pF

200kHz

8.2k

Ω

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